This invention relates to sheet feeding mechanisms for use in conveying individual documents from a print station to a stacking station.
In many types of printing and copying systems individual documents are printed or otherwise reproduced (e.g. by electrostatic copying of an original) at a first station or location (hereinafter termed the printing station), and the individual documents thus produced are serially fed to a stacking station, such as a stacking bin, where the individual documents accumulate in a stack. In many applications, it is highly desirable that the stacking station be closely adjacent the printing station for the efficient collection and distribution of the finished documents. Many different arrangements have been employed to provide a closely adjacent stacking station, and such arrangements typically include a fixed or removable tray positioned above and to the rear of the printing station into which the individual documents are serially fed and stacked automatically.
In many applications, it is highly undesirable for the sheets to collect in reverse order, which is the normal stacking mode absent any additional sheet handling mechanisms. In order to provide collated or serially arranged copies in their proper order, many improvements have been proposed and employed on the basic stacking station noted above. Such improvements include completely passive devices generally employing a stationary deflector plate against which the leading edge of each document initially bears when arriving at the stacking station and which causes the leading edge of the document to be turned upside down and deflected downwardly into the stacking tray. Such devices normally use the weight of the paper to assist in the collection of the documents in the stacking tray, and an example of such an arrangement is illustrated in U.S. Pat. No. 4,300,757. Other sheet handling mechanisms employ active elements which grasp the leading edge of the sheet as it enters the stacking station and pull the sheet typically around a one hundred eighty degree circular path provided by a platen mechanism so that the document is positively drawn into the stacking station. An example of such an arrangement is illustrated in U.S. Pat. No. 4,027,580.
Since document sheets of widely varying weights are employed in the same printing/reproducing apparatus in many applications, many stacking station arrangements with the document collation feature tend to cause jamming or crinkling of the documents, particularly when the lighter weight sheets are employed (since their resistance to crinkling is quite low). This problem is exacerbated in the passive type deflector installations which rely on the stiffness of the paper and the contact with the leading edge of the document to the defined deflection of the sheet. While it is possible to minimize this disadvantage by providing a document feed path with more gradual contours, passive deflectors are nevertheless unacceptable in those applications which require a low height profile and the efficient use of space, which is particularly true in office environment applications.
Many active devices, while adequate when used in conjunction with standard weight sheet stock, have a tendency, particularly when non-standard sheet stock is used, to wrinkle the document or tear its leading edge. While it is possible to ameliorate this problem by providing a relatively large platen having a large throat area and a large radius of curvature, this solution increases the height profile of the sheet handling mechanism, which is undesirable for low profile applications.
While the collation feature noted above is preferred in many sheet feeding applications, there are some documents for which this feature is not suited due to the requirement that the document be fed through a 180.degree. path reversal. Printed envelopes, for example, have a tendency to skew when manipulated in an active sheet feeding device, which typically results in a jamming of the mechanism. Similarly, when passed through a passive deflector the envelopes tend to accumulate in the tray in a haphazard fashion and thick envelopes tend to jam near the entry point.
With passive devices, the only practical way to defeat the collation function is to remove the deflector, which requires that the upstream printing mechanism be deactivated and is thus undesirable. Although active devices can be provided with an override mechanism to defeat the collation function, this solution requires the addition of active elements, which increases the complexity and cost of the device and increases the likelihood of mechanical failure.